1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device, an image forming apparatus, and an image forming method, and more particularly, to a fixing device for fixing a toner image on a recording medium, an image forming apparatus including the fixing device, and an image forming method using the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
In such fixing device, a heating roller and a pressing roller pressed against each other apply heat and pressure to a recording medium bearing an unfixed toner image as the heating roller and the pressing roller nip and convey the recording medium. Thus, the heat and the pressure fix the toner image on the recording medium. Specifically, the heating roller contacts the unfixed toner image on the recording medium to melt the toner. Accordingly, the melted toner, which contains resin to facilitate melting, may adhere to the surface of the heating roller. When the resin in the toner adhered to the heating roller solidifies, the solidified resin may generate asperities on the surface of the heating roller. When the heating roller having the surface asperities contacts the unfixed toner image on the recording medium to fix the unfixed toner image, the surface asperities of the heating roller may generate asperities on the fixed toner image. As a result, the fixed toner image may not have gloss.
To address this problem, a gloss application sheet may be overlaid on the image side of the recording medium which bears the unfixed toner image, so that the heating roller applies heat to the unfixed toner image on the recording medium via the gloss application sheet.
FIGS. 1 and 2 illustrate a fixing device 6R using such gloss application sheet S. In the fixing device 6R, a sheet conveyance roller 300 feeds a gloss application sheet S sent from a sheet container to a fixing nip N between a heating roller 100 and a pressing roller 200. Immediately thereafter, a recording sheet P bearing an unfixed toner image T enters the fixing nip N. Thus, while the rotating heating roller 100 and the rotating pressing roller 200 nip and convey the gloss application sheet S overlaid on the recording sheet P, heat is transmitted from the heating roller 100 to the unfixed toner image T on the recording sheet P via the gloss application sheet S to melt and fix the toner image T on the recording sheet P. Thereafter, the gloss application sheet S is separated from the recording sheet P to form a smooth, glossy toner image on the recording sheet P.
However, the fixing device 6R using the gloss application sheet S has a drawback in that, when the gloss application sheet S reaches the fixing nip N, the inherent rigidity of the gloss application sheet S may cause the gloss application sheet S to separate from the surface of the heating roller 100 near the entry to the fixing nip N, that is, at an area indicated by arrow Z in FIG. 2. As the recording sheet P moves through the position indicated by the arrow Z, the unfixed toner image T on the recording sheet P may contact the gloss application sheet S, which may disturb the unfixed toner image T. When the heating roller 100 and the pressing roller 200 fix the disturbed toner image T on the recording sheet P, a faulty toner image T may be formed on the recording sheet P.
To address this problem, the image forming apparatus may include two fixing devices, that is, a first fixing device for fixing an unfixed toner image on a recording sheet preliminarily and a second fixing device for fixing the toner image on the recording sheet finally by overlaying a gloss application sheet on the recording sheet. Alternatively, the image forming apparatus may include a single fixing device for fixing an unfixed toner image on a recording sheet preliminarily and then fixing the toner image on the recording sheet a second time when the recording sheet bearing the preliminarily fixed toner image is resent to the fixing device.
The advantage of the configuration employed by the above-described fixing devices is that the gloss application sheet does not disturb the toner image on the recording sheet because the toner image has been fixed preliminarily. However, although generally successful, such a configuration, providing both the preliminary fixing process and the final fixing process, may require more time to perform the two fixing processes, or may require two separate fixing devices or a conveyance mechanism for returning the recording sheet bearing the preliminarily fixed toner image to the fixing device for the second fixing process, thus increasing the overall size of the image forming apparatus.